This invention relates to an evacuating device for a plunger molding machine for producing molded products of ceramic batch.
With a hitherto used evacuating device for a plunger molding machine, before extruding ceramic batch supplied in a material cylinder, an evacuating cap is mounted in an air-tight manner on an end of the material cylinder to evacuate the air in the material cylinder by means of evacuating pipes so as to bring the interior of the material cylinder into a condition near to vacuum. Thereafter, a hydraulic plunger is actuated to drive its piston so as to insert a ram head into the material cylinder, thereby extruding the ceramic batch therefrom to obtain a desired molded product.
With such an evacuating device of the prior art described above, however, the interior of the material cylinder still remains under the condition near to vacuum when the piston is being retracted after completion of the extrusion of the ceramic batch from the material cylinder. As a result, part of the ceramic batch sticking to an extruding die and remaining in the material cylinder would break down, rise or partially exist in the material cylinder to form an obstruction in extruding a next ceramic batch.
Moreover, such a prior art evacuating device has a further disadvantage in that a great pulling force and a long period of time are required in order to draw the piston off and material cylinder under the condition near to vacuum.
In order to eliminate the disadvantages above described, one of the inventors of the present application proposed in Japanese Utility Model Application No. 153,575/85 an evacuating device for a plunger molding machine comprising a ram head provided with a blowdown valve which enables a piston to be simply retracted from the material cylinder.
With such an evacuating device, as shown in FIG. 1 illustrating the ram head on an enlarged scale a head 31a of the blowdown valve 31 is tapered in a frustoconical shape which is adapted to be seated on a corresponding frustoconical recess of the ram head 32 to form a seal therebetween. Therefore in using high pressure, a slight amount of ceramic material often enters between the head of the blowdown valve and the ram head probably due to the funnel-shaped recess thereby clogging an air blow aperture 33, so that the blowdown valve does not perform sufficiently. As a result, a troublesome and time-consuming operation is needed for repairing and cleaning the blowdown valve.